TMT Alternative Site:

Executive Summary December, 2016

From TMT.org

From February 2016 through October 2016, several potential sites for the TMT were evaluated to provide an alternative location for the observatory. Although Maunakea remains the preferred setting for building TMT, an alternate site is needed for the case that access to Maunakea is not possible in a timely way.

The site evaluation process was multi-dimension and included the astronomical properties of the sites for carrying out the TMT science mission, the legal arrangements for TIO to operate in the host country, processes and timescales for obtaining necessary permits, the schedule for initiation of construction, logistical issues for siting the observatory and transporting materials to the site, the cost to construct and operate the TMT at the site, and an evaluation of the risks to schedule and cost.

All of the alternative sites considered were excellent for carrying out the core science of the TMT and the interactions with potential host countries and organizations were uniformly very positive.

On October 31st, 2016, the TIO Board of Directors selected the ‘Observatorio del Roque de Los Muchachos’ (ORM), in La Palma, on the Canary Islands (Spain) as the alternate site for TMT. This decision was based on:

- The scientific importance for TMT to be located in the Northern Hemisphere and position itself as a unique facility.

• A Northern Hemisphere TMT will secure full sky coverage to the worldwide astronomy community in combination with the two other ELT projects, both to be located in the Southern Hemisphere.

- The very good quality of the ORM site, which can support TMT core science programs.

- The range of benefits provided by the ORM site including:

• Lower costs of construction and operations
• Shorter timeline to initiate construction
• Shorter timeline to ‘first-light’
• Lower project risks based on existence of support infrastructure

Our study shows that the turbulence profile above ORM is similar in character to that of Maunakea, and only second among all five sites considered regarding Adaptive Optics (AO) performances. Using the ORM turbulence profile with our own performance model for NFIRAOS, TMT’s AO facility, we could demonstrate that the TMT will perform excellently in its diffraction-limited regime at ORM.

Nevertheless, the ORM site being lower in elevation and overall warmer than Maunakea, a lower sensitivity and efficiency is expected for the TMT at this location, particularly at thermal 4 wavelengths. If TMT is built on ORM, this concern will be addressed by implementing some operations adjustments to better optimize the science output of the TMT, mainly the use of a flexible scheduling of TMT science programs (to best adapt their execution to the ambient conditions), and a prioritization of the instrumentation suites combined with an aggressive instrument development plan (e.g. focus on AO-fed instruments, development of GL AO, or early deployment of high-resolution spectrographs).

Also, because there are various stories in the community regarding dust above the ORM site, an intensive analysis has been carried out by the TMT project. This study shows that the operational and scientific risks related to dust at ORM is well within the range of the other potential sites for TMT.

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